Known cellular communication systems, such as a Universal Mobile Telecommunications System (UMTS) Radio Access Network (UTRAN), standardised by the 3rd Generation Partnership Project (3GPP), typically consist of a set of radio network controllers (RNCs), Node B, also known as Node-Bs, and mobile stations (MSs), also known as User Equipment (UEs). FIG. 1 illustrates an example of a known network topology for part of such a cellular communication system 100.
The RNCs 105 provide a connection with, for example, a Media Gateway (not shown), which acts as a translation unit between, in this case, the UMTS network and, for example, a Public Switched Telephone Network (PSTN). The RNC 105 also performs some of the higher layer processing for the UMTS network, performing functions, such as, setting up and managing radio bearers, radio resource management, supporting mobility, controlling initial access of UEs to the communication system, radio link control (RLC), etc.
The Node Bs 110 typically perform lower layer processing for the network, performing such functions as Medium Access Control (MAC), formatting blocks of data for transmission and physically transmitting transport blocks to UEs.
As can be seen in FIG. 1, Node Bs 110 are connected to an RNC 105 via an interface (Iub) 115. This interface between a Node B 110 and an RNC 105 may be a leased line, for example provided by a fixed line telecommunications provider, a microwave link, an Ethernet cable or some other form of communication link. The Node Bs are connected wirelessly to the UEs 120.
In order to conserve battery life, when a UE 120 is not involved in active connections, it is known for the UE 120 to go into an idle state, whereby the UE powers down its radio frequency circuitry (RF). When a UE 120 is in the idle state, it is important to allow the Node B to initiate a connection to the UE 120, for example when there is an in-coming call for the UE 120.
In order to achieve this, it is known for a UE 120 to periodically power up its radio circuitry in order to monitor specific channels in order to determine whether it is required to establish a connection with the network. UMTS provides two services with which a Node B is able to indicate to a specific UE 120 that it is required to establish a connection with the network. One is termed a Paging CHannel (PCH) and the other is termed a Forward Access CHannel (FACH). Details of these and other services are provided in 3GPP TS 25 221 (Universal Mobile Telecommunications System (UMTS); Physical channels and mapping of transport channels onto physical channels (TDD)) and other documents referenced therein.
The PCH is a downlink transport channel that is used to carry control information to a UE when the network does not know the specific location of the UE, i.e. the specific Node B to which the UE is attached.
In the known art, the PCH comprises two blocks: the Paging Indicator CHannel (PICH) and the PCH itself. The PICH comprises multiple indicator bits. Each UE is associated with one of the indicator bits within the PICH. Accordingly, when the UE is paged by the network using the PCH, the network sets the relevant indicator bit in the PICH. When in the idle state, the UE periodically decodes the PICH to see if the indicator bit with which it is associated has been set. If the relevant indicator has been set, the UE then reads the PCH.
Each UE has a unique identifier (UE-ID). When the network pages a UE, as previously mentioned, the network sets the relevant indicator within the PICH for that UE, and transmits the UE-ID for the UE being paged, and the relevant message within the PCH. In this manner, since more than one UE may be associated with an indicator bit in the PICH, the UE-ID enables a UE to determine whether the message is intended for that UE or not. If the PCH contains the UE-ID for the UE, the UE then reads the message, and performs the required actions. Thus, in the known art, the indicator bits are used in a paging channel to inform the UE whether it needs to turn its radio on for reading the PCH (i.e. as a battery saving mode).
The UMTS standard dictates that the PCH (and PICH) are always transmitted at a reference power level.
In a wireless communications system, the communication medium is divided into units of resource. A unit of resource can be a single code (e.g. UMTS FDD), a plurality of codes, a set of codes and timeslots (e.g. UMTS TDD), a set of timeslots (e.g. a TDD system) or a set of tones, tones and symbols or tones, symbols and timeslots (e.g. an OFDM system).
The FACH is a downlink transport channel that is used to carry control information to a UE when the system knows the location cell of the UE, e.g. the specific Node B to which the UE is attached. The FACH allows short messages to be sent from the Node B to the UE. These short messages are typically control type messages that are used, for example, to allocate physical resources to the UE, set up dedicated physical channels, etc.
The FACH is transmitted on a set of physical resources that are pre-defined and broadcast by the Node B on a Broadcast CHannel (BCH). The FACH is controlled by the RNC, which defines the codes and timeslots that are reserved for FACH transmissions. The RNC also reserves an amount of power headroom for the transmission of the FACH. When the FACH is transmitted, the RNC defines the power with which the FACH must be transmitted by the Node B. Power headroom is the amount of power that the RNC reserves for allocation of FACH resources. The power that is not reserved for ‘power headroom’ is allocated to the Node B to do with as it sees fit (for example scheduling HS-DSCH resource into). Thus, when the RNC allocates power headroom, it informs the Node B that the Node B cannot allocate that power headroom, as the power headroom is reserved by the RNC for the RNC to allocate resource into.
Unlike the PCH (and PICH), the FACH is not required to always be transmitted, and typically is only used when a message is required to be sent to a UE.
FIG. 2 illustrates a high level signal flow example 200 of an implementation for FACH transmissions. An RNC sends a ‘NODE B SETUP’ message to a Node B, instructing the Node B to reserve a certain set of codes and timeslots for FACH transmissions. This message may also reserve power headroom for use by the RNC. In UMTS, the ‘NODE B SETUP’ message for configuring the FACH transport channel is the ‘COMMON TRANSPORT CHANNEL SETUP’ message, sent to the Node B over the Iub interface 115.
The Node B 110 then periodically transmits a ‘SYSTEM INFORMATION’ message 210 on the Broadcast CHannel (BCH). This SYSTEM INFORMATION message 210 informs UEs of the physical resources used for FACH transmissions, such as the codes and timeslots in a Code Division Multiple Access (CDMA) system or the sub-carriers and timeslots in an Orthogonal Frequency-Division Multiplexing (OFDM) system. The SYSTEM INFORMATION message 210 typically also contains other broadcast information relevant to that cell, such as the network identity etc.
A UE 120 is able to receive the SYSTEM INFORMATION message 210, and configure its FACH decoding function based on the information contained within the SYSTEM INFORMATION message 210.
When the RNC 105 needs to send a message to a UE 120 using the FACH, the RNC sends a ‘SEND FACH’ message 215 to the Node B. In UMTS, the ‘SEND FACH’ message is carried using FACH frame protocol (FACH FP) messages over the Iub interface 115. The SEND FACH message 215 defines the specific code and timeslot to be used for the FACH transmission 220 from the Node B 110 to the UE 120, as well as the power level to be applied to the FACH transmission, the message contents and the identity of the UE (the UE-ID) that are to be included in the FACH transmission. The Node B 110 subsequently transmits the FACH message 220 to the UE 120, as defined by the RNC 105 in the SEND FACH message 215.
The UE 120 decodes the FACH 220 every frame, according to the definition of the FACH provided within the SYSTEM INFORMATION message 210. If the UE-ID in the FACH message 220 matches the identity that has been assigned to the UE 120, the UE 120 acts on the message contents in the FACH 220.
FIG. 3 illustrates an example of a known assignment of physical resources 300 for a single Time Division Duplex (TDD) system, and shows that one timeslot per frame is assigned for FACH usage. As previously mentioned, the physical resources 300 for the FACH are pre-assigned by the RNC 105, in the NODE B SETUP message 205, and cannot be reused by other channels. Even when the FACH is lightly used, for example when most UEs are in a connected state, receiving data traffic on a traffic channel, indicated as a High Speed Downlink Shared Channel (HS-DSCH) 305 in FIG. 3, the FACH timeslot 310 is still required to be reserved.
As will be appreciated by a skilled artisan, the fact that the FACH timeslot 310 is required to be reserved, particularly during light use of the FACH, is an inefficient use of physical resources. Consequently, the inventor of the present invention has recognised and appreciated that it is desirable for the timeslot 310 assigned to the FACH to be able to be used for other purposes when it is not required for the transmission of FACH messages. For the example illustrated in FIG. 3, the traffic channel resources, namely the HS-DSCH resources 305, comprise seven timeslots. If the timeslot 310 reserved for FACH transmissions could be utilised for a traffic channel during periods when no FACH transmissions were required to be sent, the traffic channel resources could be increased from seven timeslots to eight timeslots. This would increase the traffic channel resources by fourteen percent (14%) during those periods.
A known solution for the reuse of the FACH timeslot is for the traffic channel resources, which for the example illustrated in FIG. 3 is in the form of the HS-DSCH 305, to be used for the transmission of FACH messages. In this case, the information that would have been transmitted on the FACH transport channel is instead transmitted on the HS-DSCH transport channel. In this solution the Node B 110, as opposed to the RNC 105, controls the FACH, and the FACH timeslot of FIG. 3 becomes an HS-DSCH timeslot. In this manner, in the example of FIG. 3, the HS-DSCH 305 always comprises eight timeslots, as opposed to seven.
The HS-DSCH 305 is a shared channel that is controlled by the Node B 110. The content of the HS-DSCH 305 is allocated via Shared Control CHannels for the HS-DSCH (HS-SCCH) 315, which is a downlink physical channel that carries higher layer control information for the HS-DSCH.
The HS-SCCH 315 contains a UE-ID relating to the UE 120 for which content within the HS-DSCH 305 is intended. The UE-ID ensures that only that UE 120 for which the content of the HS-DSCH 305 is intended decodes the HS-SCCH 315 successfully. The HS-SCCH 315 also instructs the UE of those codes and timeslots that are allocated to it for the HS-DSCH 305 transmission allocated to it, and the coding and modulation of the HS-DSCH 305, e.g. code rate and modulation order, such as quadrature phase shift keying (QPSK), 16-QAM (Quadrature Amplitude Modulation), etc. Having decoded the HS-SCCH 315, the UE 120 is able to decode relevant timeslot(s) of the HS-DSCH 305 using the information contained within the HS-SCCH 315, and retrieve the relevant content.
When the HS-DSCH 305 is used to transmit the FACH, the codes and timeslots that are used for the HS-SCCH 315, as well as a FACH-ID are broadcast by the Node B, for example within the SYSTEM INFORMATION message 210 illustrated in FIG. 2. UEs that are required to monitor the FACH subsequently attempt to decode the HS-SCCH 315 using the FACH-ID. If a UE 120 is successful in decoding the HS-SCCH 315 using the FACH-ID, a FACH message is present within the HS-DSCH 315.
Having decoded the HS-SCCH 315 using the FACH-ID, the UE 120 then decodes the relevant part of the HS-DSCH 305 as allocated by the HS-SCCH 315, and using the information provided within the HS-SCCH 315.
The UE-ID of the relevant UE 120 is included with the HS-DSCH 305. If this matches the UE-ID of the UE 120 decoding the HS-DSCH 305, the UE 120 acts on the FACH message contents in the HS-DSCH 305.
When a FACH message is to be sent to a UE 120, the RNC 105 requests the Node B 110 to send a FACH to a UE 120 with a certain message. The Node B 110 then schedules the transmission of the FACH message in the HS-DSCH 305, along with any traffic data that is to be sent in the HS-DSCH 305. The Node B 110 chooses the codes and timeslots that are to be used for the HS-DSCH 305 carrying the FACH message, as well as the coding and modulation to be applied to the HS-DSCH 305. The Node B then transmits an HS-SCCH 315 that allocates the codes and timeslots on the HS-DSCH 305 for the FACH message, along with the coding and modulation used, and encodes the HS-SCCH 315 using the FACH-ID.
When there are no requests from the RNC 105 for the Node B 110 to transmit a FACH message, the Node B is able to use all of the HS-DSCH 305 resource for traffic data. In this manner, physical resources are used more efficiently, and in particular physical resources are not permanently assigned for transmitting FACH messages, in particular when no, or few, FACH messages are required to be transmitted.
However, a problem with this known technique for reusing the FACH timeslot is that it requires the use of the HS-SCCH 315 and the HS-DSCH 305 channels for transmitting FACH messages, as opposed to just a FACH channel, and as such carries a penalty in terms of power consumption.
FACH messages may be transmitted to a UE 120 when the UE is in an idle state (or alternatively in a connected state). Accordingly, the Node B 110 is unaware of the state of the transmission path between itself and the UE 120. As a result, normal power control is ineffective since the Node B 110 has no information with which to make an informed decision on the appropriate power level to use. Consequently, the power requirements must initially depend on the number of bits to be transmitted, and the coding rate, without explicit knowledge of the path loss between the UE and Node B.
A number of bits transmitted on the HS-SCCH 315 is comparable to a number of bits that are transmitted on a traditional FACH, in the order of 60 bits (57 bits for a 3.84 Mcps TDD HS-SCCH). The exact number of bits to be transmitted on the FACH depends on the particular implementation. The power requirements for HS-SCCH 315 and FACH at the start of a connection are therefore substantially the same, due to the similar number of bits to be carried on each channel, for example approximately 33% of the Node B transmit power.
A problem with this known technique for reusing the FACH timeslot is that both the HS-SSCH 315 and the HS-DSCH 305 must be used in order to transmit a FACH message, each requiring approximately 33% of the Node B transmit power, albeit in different timeslots. This is in contrast to the traditional method of transmitting FACH messages, where using the dedicated FACH timeslot only required the one timeslot. Thus, the known technique for reusing the FACH timeslot requires an additional timeslot at 33% of the Node B transmit power.
A skilled artisan will appreciate that, although the reuse of the FACH timeslot by mapping the FACH onto HS-DSCH resources provides a considerable improvement in the use of the physical resources, this increase in the power requirements is undesirable.